1. Field of the Invention
The present invention relates to a method for etching a Group III nitride semiconductor, to a method for producing a Group III nitride semiconductor crystal and to a method for producing a GaN substrate.
2. Background Art
A variety of methods for producing a semiconductor crystal are known, and examples thereof include vapor phase growth methods such as metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), and liquid phase epitaxy (LPE). One technique of LPE is a flux method employing a Na flux.
In the flux method, a molten mixture of metallic Na (sodium) and metallic Ga (gallium) is reacted with nitrogen under pressure for the growth of a GaN crystal. This method is expected to produce an inexpensive and high-quality GaN substrate because GaN crystal can be grown at a comparatively low temperature (up to 1,000° C.) and low pressure (up to 10 MPa).
In the case where a GaN crystal is grown on an underlayer (GaN or AlN) serving as a seed crystal through a flux method, the crystal properties of the GaN crystal are inherited from those of the underlayer. That is, the dislocation density of the semiconductor crystal to be grown is inherited from that of the underlayer. Therefore, the dislocation density of the grown semiconductor crystal is about 5×106/cm2 to 1×107/cm2 as same order as that of the underlayer.
A smaller dislocation density for the grown semiconductor crystal is preferred. For example, a dislocation density of 1×105/cm2 or less is preferred. Thus, in order to produce a GaN crystal having a smaller dislocation density, the dislocation density must be considerably reduced during the growth of a GaN crystal. Japanese Patent Application Laid-Open (kokai) No. 2005-12171 discloses an example of such method that a mask layer is formed on a seed crystal and GaN is laterally grown over the mask.
Japanese Patent Application Laid-Open (kokai) No. 2006-131454 (paragraph [0010]) discloses a technique for slightly growing a semiconductor crystal with the flux temperature kept at a lower temperature than the growth temperature of semiconductor crystal to be grown.
Japanese Patent Application Laid-Open (kokai) No. 2008-150239 discloses a technique for using GaN on a sapphire substrate as a template, forming a protective film on a rear surface of the sapphire substrate, and growing GaN through a flux method.
However, in the above Japanese Patent Application Laid-Open (kokai) Nos. 2005-12171, 2006-131454, and 2008-150239, the dislocation density of the GaN crystal is about 1×106/cm2. Thus, in order to produce a GaN crystal having a smaller dislocation density, the dislocation density must be considerably reduced during the growth of a GaN crystal.